1. Field of the Invention
The present invention relates to low temperature coal liquefaction, and more particularly to liquefaction of coal using a liquid clathrate.
2. Description of the Prior Art
The oldest of the modern direct liquefaction processes, dating back to about 1962, is the Solvent Refined Coal (SRC) process, developed by Spencer Chemical.
In the original process, now known as SRC-I, pulverized raw coal is mixed with a process-derived solvent and a small amount of hydrogen at high temperature and pressure. The coal dissolves; most of its ash and much of its sulfur settle out and can be removed by filtration. The resulting relatively clean liquid can be burned in that form, or it can be cooled to a tarlike solid for easier transportation and storage.
A later, modified version, SRC-II, uses more hydrogen and operates under more severe conditions of temperature, pressure, and residence time. Most of the coal is converted to liquids mainly naphtha and boiler fuel.
Recently, two 6000 ton-per-day demonstration plants--a modified SRC-I in Kentucky and an SRC-II in West Virginia--have been proposed. Conceivably, commercial-scale plants using either of these processes could be in operation by 1989 or 1990.
Another approach to coal dissolution is the Exxon Donor Solvent (EDS) process. Crushed, dried feed coal is slurried with a hydrogenated recycle solvent (the donor solvent) and fed, along with gaseous hydrogen into an upward plug-flow reactor of fairly simple design. The effluent is separated by distillation into several fractions: the recycle solvent, depleted of its hydrogen; light hydrocarbon gases; heavier distillates, boiling at up to 1000.degree. F.; and a heavy vacuum bottoms stream containing still heavier liquids, unconverted coal, and ash.
The recycle solvent is rehydrogenated catalytically in a conventional fixed-bed reactor. Bottoms are fed with steam and air to an Exxon Flexi-coking unit, which produces additional liquids and low Btu gas. In contrast to the other processes, hydrogen is obtained by steam-forming the light hydrocarbon gases.
The third direct liquefaction process currently being seriously considered for commercialization is the H-Coal process, developed by Hydrocarbon Research Inc. The H-Coal process employs no solvent. Instead, dried, crushed coal is slurried with heavy distillate from the process, pressurized, mixed with compressed hydrogen, preheated and fed to an ebullated-bed catalytic reactor.
Effluent gases are cooled to separate heavier components as liquids. Light hydrocarbons, ammonia and hydrogen sulfide are absorbed from the remaining hydrogen-rich gas, which is recompressed and recycled to the input slurry. The liquid-solid portion, containing unconverted coal, ash and oil goes to a flash separator. The lighter portions go to an atmospheric distillation unit, while the bottoms are separated with a hydrocyclone, a liquid solid separator, and a vacuum still.
All of these direct liquefaction procedures require considerable energy input and are not truly cost effective techniques.
A need therefore exists for the development of a low-energy input liquefaction process.